The present invention relates to a magneto-optical recording medium reading method and an apparatus and a magneto-optical recording medium used for execution of the method, particularly to a magneto-optical recording medium, a method, and an apparatus realizing the MSR (Magnetically Induced Super Resolution) reading.
It is requested to further increase the recording capacity of a magneto-optical disk. To improve the recording density, it is necessary to form more recording marks on a medium and thus, it is necessary to make the recording mark length smaller than the spot diameter of a laser beam and shorten an interval between the recording marks. The recording density of recording marks for magneto-optical recording or reading is restricted by the spot diameter of a light beam irradiated to a disk. It is relatively easy to form very small recording marks having a cycle equal to or less than a spot diameter. However, it is difficult to read very small recording marks exceeding the resolution of an optical system.
Therefore, an MSR reading method and an MSR medium producing the same effect as the case of decreasing a spot diameter by using the temperature distribution of a medium produced in the spot of a light beam and thereby reading recording marks out of some regions in the spot are disclosed in the Japanese Patent Application Laid-Open Nos. 1-143041 (1989) and 3-93058 (1991). The former is a magneto-optical reading method of emitting a light beam while applying a reading magnetic field to a magneto-optical disk having a multilayer structure obtained by superposing a reading layer, switching layer, and recording layer on a substrate. A temperature distribution is produced in the beam spot due to rotation of the magneto-optical disk at the time of reading and a high-temperature region and a low-temperature region are formed. In the low-temperature region, recording marks on the recording layer are transferred to the reading layer due to the exchange coupled force between the recording layer and the reading layer through the switching layer and read out. In the high-temperature region, because the exchange coupled force between the recording layer and the reading layer is lost, magnetization of the reading layer is arranged in the direction of a reading magnetic field and the recording marks on the recording layer are masked. Thereby, the recording marks are read only from the low-temperature region in the spot (FAD system) and the read resolution is improved substantially similarly to the case of focussing the beam spot.
The MSR medium disclosed in the latter Japanese Patent Application Laid-Open No. 3-93058 (1991) uses a magneto-optical reading method of emitting a light beam while applying an initial magnetic field and a reading magnetic field to a magneto-optical disk having a multilayer structure obtained by superposing a reading layer, auxiliary reading layer, intermediate layer, and recording layer on a substrate. A temperature distribution is produced in a beam spot S due to rotation of the magneto-optical disk at the time of reading. In a low-temperature region, magnetization of the reading layer is arranged in the direction of an initialization magnetic field and recording marks of the recording layer are masked (front mask). In a high-temperature region, magnetization of the reading layer is arranged in the direction of a reading magnetic field and recording marks of the recording layer are masked (rear mask). In a middle-temperature region, the recording marks of the recording layer are transferred to the reading layer through the intermediate layer and the auxiliary reading layer and read out (aperture portion). Thereby, the recording marks are read only from the middle-temperature region in the beam spot and the read resolution is improved substantially similarly to the case of focussing the beam spot (RAD double mask system).
In the case of the MSR reading system disclosed in the Japanese Patent Application Laid-Open No. 3-93058 (1991), it is necessary to arrange the magnetization of the reading layer and that of the auxiliary reading layer in the same direction by applying an initialization magnetic field with thousands of Oe by an initialization magnet. This is because the coercive force between the reading layer and the auxiliary reading layer is larger than the exchange coupled force from the recording layer through the intermediate layer.
To solve the above problem, the applicant of the present invention proposes a magneto-optical recording medium for realizing MSR reading by the RAD double mask system by applying a low reading magnetic field with hundreds of Oe without using an initialization magnet in the Japanese Patent Application Laid-Open No. 7-244877 (1995). FIGS. 1A and 1B are illustrations showing a magnetized state when reading an MSR medium proposed by the applicant of the present invention together with a film structure, in which FIG. 1A shows a magnetized state when applying a reading magnetic field in the direction opposite to the recording direction of recording marks and FIG. 1B shows a magnetized state when applying a reading magnetic field in the same direction as the recording direction of them. In FIGS. 1A and 1B, the substrate and protective layer of the medium are omitted.
As shown in FIGS. 1A and 1B, a magneto-optical disk 1 is constituted by superposing a reading layer 33, an intermediate layer 34, and a recording layer 35 on a substrate (not illustrated) in order. The reading layer 33 is a transition-metal magnetization dominant film and has a magnetization easy axis in the perpendicular direction or the superposing direction. The intermediate layer 34 is a rare-earth magnetization dominant film and has a magnetization easy axis in the in-plane direction at room temperature (10.degree. to 35.degree. C.). When the layer 34 has a predetermined temperature higher than room temperature or higher, the direction of the magnetization easy axis changes from the inward to perpendicular directions. The recording layer 35 is a transition-metal magnetization dominant film and has a magnetization easy axis in the perpendicular direction.
To form recording marks on the magneto-optical disk 1 having the above structure, a recording laser beam is emitted while applying a recording magnetic field. Information is recorded by assuming upward as the recording direction to explain the magnetized state of the magneto-optical disk 1 at the time of reading. As shown in FIG. 1A, when a reading laser beam is emitted to the magneto-optical disk 1 and a downward reading magnetic field (negative magnetic field) opposite to the recording direction is applied, the exchange coupled force between the intermediate layer 34 and the recording layer 35 is weak in a low-temperature region at the front side of the laser beam and magnetization of the intermediate layer 34 is arranged in the direction of the reading magnetic field, that is, downward. Then, the magnetized direction of the reading layer 33 is arranged upward due to the exchange coupled force between the intermediate layer 34 and the reading layer 33 to mask the magnetized direction of the recording layer 35 (front mask). Moreover, a high-temperature region is a region in which the temperature exceeds the Curie temperature of the intermediate layer 34, in which the exchange coupled force between the intermediate layer 34 and the reading layer 33 is lost. Thereby, the magnetized direction of the reading layer 33 is arranged in the downward direction of a reading magnetic field to mask the magnetized direction of the recording layer 35 (rear mask). In the middle-temperature region between the low- and high-temperature regions, the magnetized direction of the recording layer 35 is transferred to the reading layer 33 through the intermediate layer 34 (aperture portion) due to the exchange coupled force between the recording layer 35 and the reading layer 33.
Moreover, as shown in FIG. 1B, when a reading laser beam is emitted to the magneto-optical disk 1 and an upward reading magnetic field (positive magnetic field) opposite to that in FIG. 1A is applied to an irradiation region, the exchange coupled force between the intermediate layer 34 and the recording layer 35 is weak in the low-temperature region and the magnetization of the intermediate layer 34 is arranged in the direction of the reading magnetic field, that is, upward. Moreover, the magnetized direction of the reading layer 33 is arranged downward by the exchange coupled force between the intermediate layer 34 and the reading layer 33 to mask the magnetized direction of the recording layer 35 (front mask). Furthermore, the high-temperature region is a region in which the temperature exceeds the Curie temperature of the intermediate layer 34, in which the exchange coupled force between the intermediate layer 34 and the reading layer 33 is lost. Thereby, the magnetized direction of the reading layer 33 is arranged in the upward direction of the reading magnetic field to mask the magnetized direction of the recording layer 35 (rear mask). In the middle-temperature region between the high- and low-temperature regions, the magnetized direction of the recording layer 35 is transferred to the reading layer 33 (aperture portion) through the intermediate layer 34 due to the exchange coupled force between the recording layer 35 and the reading layer 33 through the intermediate layer 34. Thus, in the case of an MSR medium of the applicant of the present invention, because the magnetized direction of the intermediate layer 34 can be arranged in the direction of the reading magnetic field with hundreds of Oe, it is possible to form a front mask without using an initial magnet with thousands of Oe.
In the case of the above-described MSR medium obtained by forming a mask in the beam spot S, the mask forming range in the beam spot slightly differs depending on the direction of a magnetic field applied for reading. It is found that the inclinations of the front and rear edges of the waveform of a reading signal (reading waveform) are different from each other due to the difference in the mask forming range. FIG. 2 is an illustration showing the recording marks formed on the magneto-optical disk 1 and the waveforms of reading signals obtained by applying reading magnetic fields of negative and positive magnetic fields, which are measured by the applicant of the present invention. The recording marks whose magnetized direction is the same as that of the recording direction are shown by hatching them.
In the case of the reading waveform, the inclination of the front edge is moderate compared to that of the rear edge when reading the MSR medium by applying a negative magnetic field to the medium but the inclination of the rear edge is moderate compared to that of the front edge when reading the MSR medium by applying a positive magnetic field to the medium as shown in FIG. 2. Thus, it is found that the reading waveform has nonlinearity in either case. The front edge of the reading signal becomes steeper by applying a positive magnetic field because the magnetized direction of the intermediate layer 34 at the aperture portion is the same as the direction of the reading magnetic field at the point of time of reading the recording marks of the recording direction when applying the negative magnetic field but it is opposite to the direction of the reading magnetic field when applying the positive magnetic field (see FIGS. 1A and 1B) and thereby, the front mask is formed up to a position closer to the center of the beam spot S by applying the positive magnetic field compared to the case of applying the negative magnetic field. Moreover, the rear edge of the reading signal becomes steeper by applying the negative magnetic field because the magnetized direction of the reading layer 33 becomes the same as the magnetized direction of the intermediate layer 34 at the aperture portion at the point of time of reading recording marks when applying the negative magnetic field but it becomes opposite when applying the positive magnetic field (see FIGS. 1A and 1B) and thereby, the rear mask is formed up to a position closer to the center of the beam spot S by applying the negative magnetic field compared to the case of applying the positive magnetic field.
The inclination of the reading waveform influences jitter. As the inclination of an edge becomes moderate, the jitter increases. As described above, when the reading waveform has nonlinearity, there is a problem that the jitter of either-side edge increases and greatly influences reading characteristics. Moreover, as the recording density rises, the jitters of front and rear edges increase. In the case of a high-recording-density medium such as an MSR medium, there is a problem that a reading signal with asymmetric edges is greatly deteriorated in quality.
To solve the problem, the applicant of the present invention proposes a method of detecting the timing of an edge of a produced waveform in accordance with an obtained reading signal and reversing a reading magnetic field after detecting the edge. According to this reading method, it is possible to apply a positive magnetic field when the front edge of a recording mark is generated and a negative magnetic field when the rear edge of the mark is output and moreover, making the both edges of the reading waveform more symmetric. However, when considering the circuit delay of a signal processing circuit and the time required for reversal of a magnetic field, there is a problem that reversal of the magnetic field may not be completed before the next edge is read in the case of magnetic field modulation on the basis of the detection result of a reading signal when very small recording marks are formed at short edge intervals.